In an image forming apparatus using an electrophotographic system, such as a facsimile, a printer or a copying machine, a toner is adhered to an electrostatic latent image formed on the surface of a photosensitive member to visualize it, the visualized image is transferred to a sheet or the like and thereafter it is fixed by being heated and pressurized. In terms of achieving high image quality and colorization, as a developer, a so-called two-component developer containing a carrier and a toner is widely used.
In a development system using a two-component developer, a carrier and a toner are agitated and mixed within a development device, and the toner is charged by friction so as to have a predetermined amount. Then, the developer is supplied to a rotating development roller, a magnetic brush is formed on the development roller and the toner is electrically moved to the photosensitive member through the magnetic brush to visualize the electrostatic latent image on the photosensitive member. The carrier after the movement of the toner is left on the development roller, and is mixed again with the toner within the development device. Hence, as the properties of the carrier, a magnetic property for forming the magnetic brush, a charging property for providing a desired charge to the toner and durability in repeated use are required.
As such a carrier, a carrier which is obtained by coating, with a resin, the surface of magnetic particles such as magnetite or various types of ferrites is generally used. In the magnetic particles serving as the carrier core material, not only a satisfactory magnetic property but also a satisfactory friction charging property for the toner is required. As the carrier core material which satisfies the properties described above, carrier core materials having various shapes are proposed.
For example, in patent document 1, an electrophotographic development ferrite carrier core material is proposed which contains Sr and which has a specific shape and a magnetic property. In patent document 2, an electrophotographic development ferrite carrier core material is proposed which has a specific composition, whose lattice constant falls within a specific range and in which a surface oxide coating is formed.